Flat panel detectors are used in mammography for screening and diagnostic imaging. In both cases it is desirable to see features over the full extent of the breast, from the thickest and most dense tissue near the chest wall and in the centre of the breast, such as fibroglandular breast tissue, to the thinnest and least dense tissue near the skin boundary, such as fat and skin. A full-breast mammography image is thus obtained from x-ray signals of wide dynamic range, ranging from the maximum intensity of a nearly unobstructed x-ray beam transmitted through the skin boundary, to the substantially weakened signal transmitted through dense tissue. The presentation of a useable wide dynamic range image is achieved in film-based methods by the use of high-latitude film, which has a non-linear response and can tolerate over-exposure.
Digital mammography however, typically uses flat panel detectors that behave differently than film when exposed to varying intensities of exposure. One type of flat panel detector employs CMOS image sensors, which have an inherently linear response with poor latitude. In particular, sensors with small well size respond to overexposure by saturating. Saturation occurs when the sensor pixel values reach a constant maximum as a certain signal level is exceeded, and all data above this level is lost. CMOS image sensors however, are advantageous for mammography in that a well designed sensor will generate a low level of electronic noise, which provides enhanced contrast in dense breast tissue when compared to other types of flat panel detector.
The visibility of structures within the breast tissue is limited by the contrast-to-noise ratio (CNR) of the imaging system. The contrast of an object in a mammography image depends on the density of the object, the total x-ray attenuation provided by the object and the soft tissue in the breast, the x-ray spectrum used (photon energy and tube voltage kVp), scattered radiation and the display conditions. In addition, a non-saturating mammography exposure must be scaled to the unobstructed (air) transmission value. This can leave clinically significant areas of interest (denser tissue) underexposed, with consequently poor CNR.
The embodiments disclosed herein are directed toward overcoming one or more of the problems discussed above.